Implement CharacterData.{deleteData, insertData, replaceData}.(fixes #2190)

[lpy]

see #2190

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1332

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[bors-servo]

saw approval from Ms2ger
at https://github.com/lpy/servo/commit/fd9541357f3b271577f87a4761759545604c803e

[bors-servo]

merging lpy/servo/issue2190 = fd95413 into auto

[bors-servo]

lpy/servo/issue2190 = fd95413 merged ok, testing candidate = 3c175c7

[bors-servo]

all tests pass:
success: http://servo-buildbot.pub.build.mozilla.org/builders/linux/builds/1260
success: http://servo-buildbot.pub.build.mozilla.org/builders/mac/builds/1214

[bors-servo]

fast-forwarding master to auto = 3c175c7